1. Field of the Invention
The present invention relates to a thermal transfer sheet.
2. Description of Related Art
There are known various types of thermal transfer sheets for transferring a transfer layer onto a transfer receiving article. For example, thermal transfer sheets suggested in Patent Literatures 1 to 3 are known, such as: (i) a thermal transfer sheet in which a thermally fusible ink layer as the transfer layer is provided on one surface of the substrate, (ii) a thermal transfer sheet in which a receiving layer as the transfer layer is provided on one surface of the substrate (it is referred to as an intermediate transfer medium, occasionally), (iii) a thermal transfer sheet in which a protective layer (it is referred to as an exfoliate layer, occasionally) as the transfer layer is provided on one surface of the substrate (it is referred to as a protective layer transfer sheet, occasionally), and (iv) thermal transfer sheets including an appropriate combination of these structures, for example, a thermal transfer sheet in which a transfer layer of a layered structure including an exfoliate layer and a receiving layer layered in this order from the side of the substrate is provided on one surface of the substrate and a thermal transfer sheet in which a thermally fusible ink layer and a protective layer are provided on the same surface of the substrate so as to be layered in parallel on the substrate across the surface of the substrate, as being frame sequentially. The transfer layer of these thermal transfer sheets is transferred onto a transfer receiving article by superposing such a thermal transfer sheet on the transfer receiving article and heating the other side of the substrate by a heating device such as a thermal head and a heating roller.
The market is now highly demanding printers highly suitable for high-speed printing. Energy applied to a thermal transfer sheet (it is referred to as thermal energy, application energy or the like, occasionally) when a transfer layer is transferred onto a transfer receiving article inside a printer has been steadily increasing. As printers used for transferring the transfer layer of thermal transfer sheets, there are known hot release-type printers that apply energy to a thermal transfer sheet to melt or soften the transfer layer and release only the transfer layer transferred on a transfer receiving article from the thermal transfer sheet before this transfer layer solidifies and cold release-type printers that release only the transfer layer transferred on the transfer receiving article from the thermal transfer sheet after the transfer layer has solidified. Transfer of the transfer layer onto a transfer receiving article is carried out by applying energy to the thermal transfer sheet while the transfer receiving article and the transfer layer of the thermal transfer sheet are kept in close contact to each other to transfer the transfer layer onto the transfer receiving article and releasing the transfer layer transferred on the transfer receiving article from the thermal transfer sheet. Incidentally, in the case where a transfer receiving article and the thermal transfer sheet are thermally fused to each other when the transfer layer of the thermal transfer sheet is transferred onto the transfer receiving article, specifically, in the case where the transfer receiving article and the thermal transfer sheet adhere to each other to such an extent that it is not possible to release the transfer layer transferred on transfer receiving article from the thermal transfer sheet, for example, in the case where the transfer layer and the substrate are thermally fused to each other unintentionally when the thermal transfer sheet in which the transfer layer is provided directly on the substrate is used to transfer the transfer layer onto the transfer receiving article, problems are likely to occur such as rupture of the thermal transfer sheet inside the printer, conveyance failures of the thermal transfer sheet inside the printer (it is referred to as a jam, occasionally), and the like. Particularly, as energy applied to the thermal transfer sheet is increased when the transfer layer is transferred, the occurrence frequency of thermal fusion between the transfer receiving article and the thermal transfer sheet and conveyance failures caused by thermal fusion tends to increase. These problems tend to occur more frequently in hot release-type printers than in cold release-type printers.
Energy applied to the thermal transfer sheet when the transfer layer is transferred onto the transfer receiving article also depends on heating devices and the like. It is thus desirable that a thermal transfer sheet used when a transfer layer is transferred onto a transfer receiving article have a good release property (hereinafter, it is referred to as the release property of the transfer layer) when only the transfer layer transferred on the transfer receiving article is released from the thermal transfer sheet, not only in the case where the energy applied to the thermal transfer sheet is high but also in the case where the energy applied to the thermal transfer sheet is low when the transfer layer is transferred onto the transfer receiving article, that is, regardless the amount of the energy applied to the thermal transfer sheet when the transfer layer is transferred onto the transfer receiving article, in other words, in a wide applied energy field. Under the present circumstances, however, the release property of the transfer layer has not been satisfied in a wide applied energy field, and there is a room for improvement on this respect.
Although various studies have been made to suppress thermal fusion between a transfer receiving article and a thermal transfer sheet, there is a room for improvement on measures for thermal fusion between a transfer receiving article and a thermal transfer sheet, which may occur when the transfer layer of the thermal transfer sheet is transferred onto the transfer receiving article by applying high energy to the thermal transfer sheet.